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Sum of digits bounding

Source: Kvant Magazine No. 2 2024 M2783

May 2, 2024
number theorysum of digits

Problem Statement

The sum of the digits of a natural number is k.k{}. What is the largest possible sum of digits for [*] the square of this number; [*]the fourth power of this number, given that k4.k\geqslant 4.
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